Device for the contactless and nondestructive testing of surfaces

ABSTRACT

A device for the contactless and nondestructive testing of a surface by measuring the infrared radiation thereof has one or more incoherent electromagnetic radiation sources ( 1 ) and a detector ( 14 ) arranged on a detection axis ( 9 ), wherein the radiation sources ( 1 ) are arranged at a radial distance from the detection axis ( 9 ), at a distance from a testing area ( 7 ). In this arrangement, a pulsed or intensity-modulated excitation radiation ( 2 ) can be generated by these radiation sources ( 1 ) and applied to the surface ( 6 ) to be tested in the testing area ( 7 ) at an inclination to the detection axis ( 9 ) in the testing area ( 7 ). The detection radiation emitted by a measuring area ( 8 ) of the surface ( 6 ) to be tested can be fed to the detector ( 14 ), wherein the detector ( 14 ) is arranged on the detection axis ( 9 ) further away spatially from the testing area ( 7 ) than the radiation sources ( 1 ). Furthermore, an imaging device ( 10, 12 ) is provided on the detection axis ( 9 ) for creating an image of the testing area ( 7 ) on the measuring area of the detector ( 14 ) that is arranged between the radiation sources ( 1 ).

This is a Continuation of application Ser. No. 13/642,956 filed Oct. 23,2012, claiming priority based on International Application No.PCT/CH2011/000097 filed May 2, 2011, claiming priority based on SwissPatent Application No. 667/10, filed May 3, 2010, the disclosures ofwhich are incorporated herein in their entirety by reference.

TECHNICAL FIELD

The invention relates to a device for the contactless andnon-destructive testing of a surface by measuring its infraredradiation.

STATE OF PRIOR ART

The contactless testing of surfaces based on the generation andmeasurement of transient or periodic heating and cooling processesrequires an excitation source for heating the surface to be tested aswell as an infrared detector which measures the infrared radiationemitted from the heated surface. This method is called photothermy ifelectromagnetic radiation in the ultra-violet, optical or infrared rangeis used for excitation. The excitation radiation may for example comefrom the back of a surface, whilst the infrared radiation emitted fromthe surface is recorded at the front, as described by Parker et al.(1961).

In a configuration described by Leung and Tam (1984) excitation anddetection take place on the same side of the surface. This configurationoffers the advantage of also allowing surfaces to be tested which areaccessible from one side only, and thus a much more varied range ofapplications.

Normally lasers are used as excitation sources for photothermal testssuch as described by Eyrer and Busse (1984). The advantages of laserradiation for excitation are easy direction of the rays as well as highenergy density due to ray bundling. These advantages permit locallyprecise positioning and precise metering of the excitation radiation.However, for applications outside a controlled protective environment,laser excitations suffer from major disadvantages. Photo-thermal testingrequires powerful lasers (for example with 14 W optical output withpulse lengths of up to 0.1 s, see short documentation of Paintcheckermodular of Messrs. Optisense) in order to achieve a measurable increasein temperature, which is dangerous to humans and animals (such lasersfall into laser classes 3 or 4). Adequate precautions are thereforenecessary and the measuring system can only be operated by trainedpersonnel. Moreover suitable laser excitations are relatively expensive.Therefore increased expenditure has to be expected in conjunction withthe purchase and operation of a testing equipment with laser excitation.

In WO 95/16907 a device for photothermal testing of surfaces with laserexcitation is described. The excitation beam is focussed onto a mirrorwhich deflects the excitation beam through an opening in the sensorcollecting lens onto the surface of the test piece. Apart from thedisadvantages connected with using a laser source this arrangement has afurther disadvantage: due to the shadow thrown by the reflecting mirrorand due to the hole in the collecting lens part of the infraredradiation to be captured and emitted by the test piece gets lost.

As with laser sources alternative electromagnetic radiation sources suchas flash lamps must comprise a sufficiently high energy density in orderto achieve a measurable increase in temperature in the testing area ofthe surface to be tested. If required several flash lamps may be used inorder to deposit as much energy as possible in the testing area, forexample if the radiation behaviour is spread out. Due to their spatiallywidespread radiation behaviour the major part of the excitationradiation is lost. Therefore flash lamps are not very suitable asthermal excitation sources for the photo-thermal test procedure.

In WO 98/05949 a device is described in which at least four flash lampsof 6 kJ optical energy each are to be used for excitation of a testpiece. With regard to common flash durations of 5 ms of a flash lampthis corresponds to a short-term optical power of 1.2 MW, wherein,however, only a fraction of the output is deposited in the testing area.The major part of the optical radiation does not fall within in thetesting area. In order not to lose the part emitted from the backaltogether, reflectors are placed behind the lamps. The infrared sensormust then be placed laterally of the excitation source(s) since noinfrared radiation from the measuring area is detectable behind theexcitation source(s). This lateral placement in turn has decisivedisadvantages: the measuring device is spatially extended due to theparallel arrangement, and the radiation paths for excitation anddetection cannot progress collinearly. The effect is that either theexcitation radiation hits the surface to be tested at a flattened angle,or the detection radiation is detected at a flattened angle. Theflattened angle leads to losses during heating of the surface or/and toa more extensive measuring arrangement during detection of its detectionradiation.

A device with the characteristics of the preamble of claim 1 is knownfrom the DE 198 30 473, where the infrared radiation sources are coveredand exposed at intervals by a rotating covering unit.

DESCRIPTION OF THE INVENTION

Based on this state of the art the invention relates to a device for thecontactless and non-destructive testing of surfaces by testing activelyexcited thermal processes, which can be manufactured at reduced cost andwhich is very easy and practical in use.

This and further requirements and advantages of the present inventionare met by the features of patent claim 1.

The core of the invention consists in an arrangement of an excitationsource and a reflection device for the excitation radiation emitted bythe excitation source, wherein the reflection device bundles theexcitation radiation emitted into wide volumes of space and applies theexcitation radiation to a testing area of a surface to be tested throughan opening of the reflection device and in that a radiation detectordetects the infrared radiation (from now on called detection radiation)from the testing area generated by heating through the same opening.

In order to achieve as homogenous an illumination of the testing area aspossible an annular excitation source is used in an especially preferredembodiment of the invention.

According to a further embodiment of the invention an infraredcollecting lens or infrared mirror optics is placed in a recess in thecentre of the excitation source, which focuses the detection radiationupon the infrared sensor.

This arrangement results in a number of advantages compared to knownstate-of-the-art devices. Since the arrangement described offers aspatially wide area of intersection between excitation radiation anddetection radiation, the device is robust in relation to distancechanges and tilting between the arrangement and the surface to betested. In the embodiment of the invention in which the detectionradiation is directed from the testing area to the detector through arecess in the excitation source, the device can be constructed in a verycompact manner and radiation losses are minimised because both theexcitation radiation and the detection radiation extend largelyvertically to the testing area. In one embodiment of the invention thereflecting housing is a truncated-cone-shaped funnel from the narrowedopening of which emerges the excitation radiation and detectionradiation. Due to the tapered shape the excitation radiation is bundled.As a result maximum energy efficiency is obtained which is of advantagefor a compact construction and which minimises any heat losses of thedevice.

Advantageous embodiments and further developments are proposed in thedependent sub-claims.

In a preferred embodiment of the invention an annular flash lamp orseveral flash lamps arranged in a ring are used for excitation.Gas-discharge lamps are particularly suited as excitation sources due totheir high efficiency and the achievable high densities. The spectrum oflight generated by Xenon gas-discharge lamps substantially correspondsto the radiation curve by Planck at a temperature of approximately5000K. The major part of the excitation radiation generated is producedin the visible spectral range between 400 nm and 800 nm, wherein anon-negligible portion of the generated excitation radiation is producedin the infrared wavelength range. The broad excitation spectrum isespecially advantageous because electromagnetic radiation is absorbedindependently of the spectral properties of the surface to be tested. Ina preferred embodiment the incoherent light source consists of one ormore LEDs. Especially advantageous is a ring-shaped arrangement of LEDs.The use of LEDs offers the advantage that electromagnetic radiation canbe generated in a defined spectral range. However, only surfaces with aspectral behaviour can be tested, which permit absorption of theelectromagnetic excitation radiation generated by the LEDs. Thisrestriction may be compensated, for example, by a combination ofdifferently coloured LEDs.

Depending upon the range of spectral sensitivity of the infrared sensorused, there is the possibility that part of the excitation radiationreflected by the test piece is detected by the infrared sensor whichleads to an undesirable spectral superimposition of the excitationradiation by the heat radiation generated through heating. In order toavoid this spectral superimposition it is advantageous to place a filterdevice immediately in front of the flash lamp(s) for separating infraredportions of the spectrum from the radiation generated by the flashlamp(s). A preferred embodiment of the filter device consists of ahousing transparent for the excitation radiation and filled with aliquid (for example water) which has an absorbing effect in the spectralsensitivity range of the infrared sensor. In an especially preferredembodiment of the invention (FIG. 8) the excitation radiation isembedded in the filter medium. If the filter medium used is a liquid,this can be pumped through a heat exchanger in a preferred embodimentand cooled, thereby ensuring active cooling of the excitation source.

SHORT DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention will now be described by way ofthe drawings which merely serve as an explanation and are not to beregarded as restrictive. In the drawings:

FIG. 1 shows a schematic sectional view through a device according to afirst embodiment of the invention;

FIG. 2 shows a schematic sectional view through a device according to asecond embodiment of the invention;

FIG. 3 shows a schematic sectional view through a device according to athird embodiment of the invention;

FIG. 4 shows a schematic sectional view through a device according to afourth embodiment of the invention;

FIG. 5 shows a schematic sectional view through a device according to afifth embodiment of the invention;

FIG. 6 shows a schematic sectional view through a device according to asixth embodiment of the invention;

FIG. 7 shows a schematic sectional view through a device according to aseventh embodiment of the invention;

FIG. 8 shows a schematic sectional view through a device according to aneighth embodiment of the invention;

FIG. 9 shows a schematic sectional view through a device according to aninth embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 1 to 9 each show a schematic sectional view through a deviceaccording to a first embodiment of the invention. Identical or similarfeatures in all figures are marked with the same reference symbols.

There is at least one radiation source 1 which at least on two oppositesides, as shown in FIG. 1, focuses excitation radiation 2 onto a testingarea 7 of a surface 6 to be tested. In the very schematic parallelrepresentation of excitation radiation 2 the function of theconcentrator described below is anticipated. The excitation radiation 2is generated by these radiation sources 1, which in particular may beone or more flash lamps arranged in a ring. The cross-section in FIG. 1would thus be a rotation-symmetrical representation about thelongitudinal axis of the device as indicated by arrow 9.

The flash lamps 1 are arranged in a torus open on one side of areflection surface 3, which torus, at its one-sided opening, comprises aring-shaped filter 4.

The filter device 4 is designed to separate infrared spectral portionsof the light of flash lamps 1.

The excitation radiation 2 passes through these filters and is directedby the device described for bundling and directing the excitationradiation and called concentrator in the following, onto the surface 7to be tested. In a preferred embodiment the concentrator, as shown inFIG. 1, comprises a conically narrowing reflection surface 3, whereinthe angle of aperture of the cone is between 10° and 80°. Thisarrangement of the reflection surfaces 3 directs the light especiallyefficiently to the exit opening 5. In other words, the reflectionsurfaces 3 in this area form a hollow truncated cone open on both sides.On one side this comprises an exit opening 5 which is preferablycircular or framed by a polygon course, on the opposite side itcomprises the larger passing-through surface closed off by the circularfilter 4 and an inner collecting lens.

The truncated cone-shaped reflection surfaces may also comprise theshape of a pyramid stump or a tapering hollow body limited by trapezes,the two openings of which are then formed by polygon courses. Atruncated-cone shape is particularly meaningful for a flash lamp shapedas an annular circle, a pyramid stump is suitable for two or four flashlamps 1, a polygon course of an octagon for four or eight flash lamps 1.All these hollow, mirror-symmetrical (about axis 9) reflective bodiestapering from a large opening on the side of the flash lamp(s) 1 indirection of the testing area 7 and the opening 5 are called here atruncated cone. The opening 5 determining the testing area 7advantageously comprises an aperture of 0.1 to 10 centimeters which fora non-round opening 5 is defined by the diameter between individualopposite portions of the polygon courses.

In a preferred embodiment the reflection surface of the concentratorconsists, at least towards the inside, of a highly reflective materialwith a reflectivity of 20% to 100% which directs the light towards theexit opening 5 almost without any losses.

In a further preferred embodiment this highly reflective materialconsists of aluminium.

In a further preferred embodiment this highly reflective materialconsists of stainless steel which is particularly favourable with regardto manufacturing cost. If the inside surface is polished, the portion ofexcitation radiation directed to the testing area 7 can be furtherincreased.

In a further preferred embodiment this highly reflective materialconsists of gold which comprises a particularly small emissivity in theinfrared spectral range.

Due to the small emissivity of the gold surface the superimposition ofthe measured infrared radiation from the testing area and the insidesurfaces of the concentrator is particularly small. In other words, theportion of heat radiation emitted by the reflection surfaces 3themselves and which can also be recorded by the detector, is verysmall.

In order to generate a well defined measuring area, the detector 14, ina preferred embodiment, is mapped onto the surface by means of asuitable device, as described below.

The mapping device shown in FIG. 1 consists of two lenses 10 and 12arranged on the central axis along arrow 9. Preferably the collectinglens 10 facing the testing area is not located in front of the planewhich is defined by the orifice of the cone of the reflection surfaces3. In other words, this lens 10 lies in the shade of the light portionsof flash lamps 1 exiting through the filter elements 4. The collectinglens 10 collimates the detection radiation emitted by the testing area7, which is symbolised by the arrow 9 on the longitudinal axis of thedevice. A further focussing lens 12 has the task of guiding thecollimated radiation 13 onto the detector. In a particularly preferredembodiment the distance of the collecting lens 10 from the measuringarea 7 or focussing lens 12 from the detector 14 is equal to the focaldistance of the two lenses 10 or 12. The area of the parallel bundle ofrays 11 is chosen so as to ensure that the detector 14 is arrangedsafely behind the reflection torus and thus cannot absorb any indirectradiation from the inside or outside surfaces of the reflection surfaces3, and this is additionally ensured by a corresponding aperture openingnot shown in FIG. 1 about the waist of the focussing cone 13.

FIG. 2 shows an alternative mapping device consisting of only one lens10. In an arrangement to be preferred this lens 10 is positioned in themiddle between the measuring area 7 and the detector 14, wherein itsfourfold focal distance corresponds the distance between measuring area7 and detector 14. The other features of this embodiment correspond tothose of the embodiment shown in FIG. 1. In particular the annular flashlamp 1 may be replaced by a plurality of individual flash lamps 1 in thereflection torus open on one side, or provision may be made forindividual cylinders open on one side with inserted individual flashlamps 1 which would be possible in the representation of FIG. 2.

In a further embodiment to be preferred the detector 14 is asemiconductor detector. Semiconductor detectors are particularlysensitive and have short response times. In a particular embodiment thissemiconductor detector has a sensitivity range between 2 micrometers and20 micrometers and response times between 1 nanosecond and 1 second.Peltier or nitrogen-cooled semiconductor detectors are to be especiallypreferred because of their low-noise behaviour. The use of Bolometers asdetectors for the detection radiation permits an especiallycost-effective manufacture.

The part of the reflection device 3 facing away from the testing areamay be implemented in an especially preferable further arrangement bycurved reflection surfaces such as shown in FIG. 3. In this way thebackwards radiated portion of the excitation radiation can be directedparticularly efficiently to the testing area, as illustrated by theradiation progression indicated about arrow 2 of the excitationradiation.

The embodiment shown in FIG. 4 comprises curved reflection surfaces 3which direct the excitation radiation 2 particularly efficiently to thetesting area, as illustrated by the radiation progression indicatedabout arrow 2 of the excitation radiation. The radius of curvaturedecreases in cross-section from a tangential constant progression at thereflection torus in the area of filters 4 down to a reflection wallextending vertically to arrow 9 in the area of opening 5 which, in theend, then defines exactly this opening 5.

With a further preferred embodiment shown in FIG. 5 the concentratorcomprises reflection surfaces 3 which can be approximated using apolynomial function, in particular a function such as f(x)=x*x. Thesedirect the light especially efficiently to the exit opening 5.

With a further preferred embodiment the concentrator has hyperbolicreflection surfaces 3 in cross-section. These also direct the lightespecially efficiently to the exit opening 5.

With a further preferred embodiment the concentrator has ellipticalreflection surfaces 3 in cross-section. These direct the lightespecially efficiently to the exit opening 5. In particular, a tiltingof the drawn ellipsoid by between 10°-80° relative to axis 9 isespecially preferred, wherein one of the two focal points is at thelocation of the excitation source 1 and the other focal point is at thelocation of the measuring area 7.

The embodiment shown in FIG. 5 comprises reflection surfaces 3 whichreflect forwards and backwards.

In the embodiment shown in FIG. 6 the light is directed, in addition toreflection surfaces 3, to the testing area through curved surfaces inthe area of filters 4. The curved surfaces may either be attached lensesor, on the other hand, a curvature of the surface of the filter medium4. The focal distance should be chosen such that the waist diameter ofthe image of the light source elements (gas discharge section, filamentetc.) of the excitation radiation roughly corresponds to the size ofopening 5 and lies typically between 0.01 m and 2 m depending on thedistance to the surface 6 to be tested.

In the embodiment shown in FIG. 7 the excitation radiation is directedthrough these attached lenses solely on the basis of optical refractionat the same, causing a curvature of the surface of the filter medium 4to the testing area 5.

In the embodiment shown in FIG. 8 the flash lamps 1 are embeddeddirectly into the filter medium 4.

The embodiment shown in FIG. 9 represents a combination of especiallypreferable features with curved reflection surfaces 3 of the rearwardand forward part of the reflection device and an electromagneticexcitation source 1 embedded into the filter medium 4. This embodimentpermits especially efficient guiding of the electromagnetic excitationradiation 1 and a compact construction. The filter medium used may bewater, in particular. The water is held in a water-tight implementationof the reflection device closed off by a pane, preferably a glass pane,which is transparent for the excitation radiation. The connectionbetween reflection device and pane may be preferably effected by anadhesive or a rubber seal. Pressing the pane against the rubber seal maypreferably be effected by a screw connection. A continual exchange ofwater for use as a coolant may be effected in that an inflow of watervia a breakthrough is preferably effected at the bottom and in that theoutflow is preferably effected via a breakthrough at the top.

In a further preferred embodiment the detection radiation 9, 11, 13bundled by the imaging device may be fed into an optical conductor andvia this optical conductor to a detector.

The various embodiments of the invention permit determination of thelayer thickness of coatings as well as of their thermal properties suchas diffusivity and effusivity or thermal conductivity and heat capacity,as well as the determination of adhesive properties of coatings.

LIST OF REFERENCE SYMBOLS

-   -   1 radiation source    -   2 excitation radiation    -   3 reflection surface    -   4 filter device    -   5 opening    -   6 surface to be tested    -   7 testing area    -   8 measuring area    -   9 detection radiation    -   10 collecting lens    -   11 parallel ray bundle    -   12 focussing lens    -   13 focussing cone    -   14 detector

The invention claimed is:
 1. A device for the contactless andnon-destructive testing of a test surface by measuring the infraredradiation thereof, comprising: one or more incoherent electromagneticradiation sources, a detector providing and arranged on a detection axisand comprising a measuring area, a testing area defining an area to bemeasured of the test surface, and an imaging device arranged on thedetection axis for mapping the testing area onto the measuring area ofthe detector, wherein the radiation sources are arranged at a radialdistance from the detection axis at a distance from the testing area,wherein the radiation sources are adapted to generate a pulsed orintensity-modulated excitation radiation which can be directed onto thesurface to be tested in the testing area, wherein, in response toradiation impinging onto the surface to be tested in the testing area,detection radiation is emitted by the measuring area of the surface tobe tested and fed to the detector, wherein the radiation sources arearranged in a housing having the form of a torus or polygon course openon one side towards the testing area, wherein the excitation radiationfrom the radiation sources is fed to the testing area at an inclinationto the detection axis, wherein the detector on the detection axis isarranged spatially further away from the testing area than the radiationsources, wherein the imaging device is arranged between the radiationsources, wherein one or more optical filter devices are arranged betweenthe radiation sources and the testing area, through which the excitationradiation is guidable to the testing area, wherein the filter devicesare designed to absorb one or more spectral ranges of the generatedexcitation radiation, wherein the one or more optical filters areprovided as a ring-shaped or polygon-shaped filter at the one-sidedopening of the housing.
 2. The device according to claim 1, wherein theopening determining the testing area comprises an aperture of between0.1 and 10 centimeters.
 3. The device according to claim 1, wherein theimaging device directs the detection radiation passing through theopening of the reflection device and emitted by the testing area to thedetector in the measuring area defined by the imaging device.
 4. Thedevice according to claim 1, wherein the imaging device comprises acollecting lens which collects the detection radiation emitted by themeasuring area and converts it into a parallel bundle of rays, thecollecting lens being arranged between the torus shaped or polygoncourse shaped housing, so that the detector is arranged safely behindthe reflection housing and thus cannot absorb any indirect radiationfrom the inside or outside surfaces of the reflection surfaces.
 5. Thedevice according to claim 1, wherein the imaging device comprises afocusing lens which maps the detection radiation onto the detector.